Method for making a light emitting diode having three dimensional nano-structures

ABSTRACT

A method for making a LED comprises following steps. A substrate having a surface is provided. A first semiconductor layer, an active layer and a second semiconductor pre-layer is formed on the surface of the substrate. A patterned mask layer is applied on a surface of the second semiconductor pre-layer. A number of three-dimensional nano-structures is formed on the second semiconductor pre-layer and the patterned mask layer is removed. The substrate is removed and a first electrode is formed on a surface of the first semiconductor layer away from the active layer. A second electrode is formed to electrically connect with the second semiconductor pre-layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims all benefits accruing under 35 U.S.C. §119 from China Patent Application No. 201210089073.3, filed on Mar. 30, 2012 in the China Intellectual Property Office, the disclosure of which is incorporated herein by reference. This application is related to applications entitled, “METHOD FOR MAKING SOLAR CELLS”, filed Dec. 27, 2012 Ser. No. 13/727,988, “SOLAR CELLS”, filed Dec. 27, 2012 Ser. No. 13/727,999, “WHITE LIGHT EMITTING DIODES”, filed Dec. 27, 2012 Ser. No. 13/728,006, “METHOD FOR MAKING LIGHT EMITTING DIODES”, filed Dec. 27, 2012 Ser. No. 13/728,018, “LIGHT EMITTING DIODES”, filed Dec. 27, 2012 Ser. No. 13/728,031, “LIGHT EMITTING DIODES”, filed Dec. 27, 2012 Ser. No. 13/728,035, “LIGHT EMITTING DIODES”, filed Dec. 27, 2012 Ser. No. 13/728,054, “LIGHT EMITTING DIODES AND OPTICAL ELEMENTS”, filed Dec. 27, 2012 Ser. No. 13/728,063, and “METHOD FOR MAKING LIGHT EMITTING DIODES AND OPTICAL ELEMENTS”, filed Dec. 27, 2012 Ser. No. 13/728,076.

BACKGROUND

1. Technical Field

The present disclosure relates to, methods for making a light emitting diode (LED).

2. Discussion of Related Art

LEDs have higher energy conversion efficiency, higher radiance (i.e., they emit a larger quantity of light per unit area), longer lifetime, higher response speed, generate less heat, and have better reliability than conventional light sources. Therefore, LED modules are widely used as light sources in optical imaging systems, such as displays, projectors, and so on.

A conventional LED commonly comprises an N-type semiconductor layer, a P-type semiconductor layer, an active layer, a first electrode, and a second electrode. The active layer is located between the N-type semiconductor layer and the P-type semiconductor layer. The second electrode is located on the P-type semiconductor layer. The first electrode is located on the N-type semiconductor layer. Typically, the second electrode is transparent. In operation, a positive voltage and a negative voltage are applied respectively to the P-type semiconductor layer and the N-type semiconductor layer. Thus, the holes in the P-type semiconductor layer and the electrons in the N-type semiconductor layer can enter the active layer and combine with each other to emit visible light.

However, the efficiency of LEDs is limited by several factors comprising the high refractive index of the P-type semiconductor layer and/or the N-type semiconductor. Therefore, an external quantum efficiency of LEDs is low.

What is needed, therefore, is to provide a method for making a light emitting diode and an optical element, which can overcome the above-described shortcomings.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the embodiments can be better understood with references to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the embodiments. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.

FIG. 1 is a schematic view of one embodiment of a light emitting diode.

FIG. 2 is a schematic view of a second semiconductor layer of the light emitting diode shown in FIG. 1.

FIG. 3 shows a scanning electron microscope (SEM) image of the second semiconductor layer shown in FIG. 2.

FIG. 4 shows a light extraction schematic view of the second semiconductor layer shown in FIG. 2.

FIG. 5 shows light extraction intensity curves of an embodiment of light emitting diode and a conventional light emitting diode respectively.

FIG. 6 shows a flowchart of one embodiment of a method for forming a light emitting diode.

FIG. 7 shows a process of one embodiment of a method for forming a number of first three-dimensional nano-structures on a second semiconductor pre-layer.

FIG. 8 shows a process of one embodiment of a method for etching a second semiconductor pre-layer.

FIG. 9 is a schematic view of another embodiment of a light emitting diode.

FIG. 10 shows a flowchart of another embodiment of a method for forming a light emitting diode.

DETAILED DESCRIPTION

The disclosure is illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean at least one.

Referring to FIG. 1, one embodiment of an LED 10 is provided. The LED 10 comprises a first semiconductor layer 110, an active layer 120, a second semiconductor layer 130, a first electrode 140, and a second electrode 150. The first semiconductor layer 110, the active layer 120 and the second semiconductor layer 130 are stacked in that order and are located on a surface of first electrode 140. The first electrode 140 is electrically connected to the first semiconductor layer 110. The second electrode 150 is electrically connected to the second semiconductor layer 130. A surface of the second semiconductor layer 130, away from the active layer 120, is the light emitting surface of the LED 10.

The first semiconductor layer 110 can be located on the surface of first electrode 140. The first semiconductor layer 110 can be an N-type semiconductor or a P-type semiconductor. A material of the N-type semiconductor can comprise N-type gallium nitride, N-type gallium arsenide, or N-type copper phosphate. A material of the P-type semiconductor can comprise P-type gallium nitride, P-type gallium arsenide, or P-type copper phosphate. The N-type semiconductor can be configured to provide electrons, and the P-type semiconductor can be configured to provide holes. A thickness of the first semiconductor layer 110 can range from about 1 μm to about 5 μm. In one embodiment, the first semiconductor layer 110 is an N-type gallium nitride.

The active layer 120 can be located on a surface of the first semiconductor layer 110 away from the first electrode 140. In one embodiment, the active layer 120 covers the entire surface of the first semiconductor layer 110, away from the first electrode 140. The active layer 120 can be a photon excitation layer. The active layer 120 can be one of a single layer quantum well film, or multilayer quantum well films. A material of the active layer 120 can be GaN, GaInN, AlGaInN, GaSn, AlGaSn, GaInP, or GaInSn. A thickness of the active layer 120 can range from 0.01 μm to about 0.6 μm. In one embodiment, the active layer 120 has a thickness of about 0.3 μm and comprises a layer of GaInN and a layer of GaN stacked with the GaInN layer.

The second semiconductor layer 130 can be located on a surface of the active layer 120, away from the first semiconductor layer 110. In one embodiment, the second semiconductor layer 130 covers the entire surface of the active layer 120. A thickness of the second semiconductor layer 130 can range from about 0.1 μm to about 3 μm. The second semiconductor layer 130 can be an N-type semiconductor layer or a P-type semiconductor layer. Furthermore, the type of the second semiconductor layer 130 is different from the type of the first semiconductor layer 110. A surface of the second semiconductor layer 130 away from the active layer 120 can be used as a light emitting surface of the LED 10. In one embodiment, the second semiconductor layer 130 is a P-type gallium nitride doped with Mg and the thickness of the second semiconductor layer 130 is about 0.3 μm.

Referring to FIG. 2 and FIG. 3, the second semiconductor layer 130 can comprise a body 132 and a number of the first three-dimensional nano-structures 134. The first three-dimensional nano-structures 134 can be located on a surface of the body 132 away from the active layer 120.

The first three-dimensional nano-structures 134 can be linear protruding structures. The linear protruding structures can protrude out of the surface of the body 132 to form an integrated structure. The linear protruding structures can be uniformly distributed on the surface of the body 132 and spaced from each other. The linear protruding structures can be uniformly distributed on the surface of the body 132 to form an array. The linear protruding structures in the array can be substantially equidistantly arranged, concentric circularly arranged, or concentric rectangularly arranged. In one embodiment, the linear protruding structures are substantially equidistantly arranged.

The linear protruding structures can be arranged in a straight line, a curvy line, or a polygonal line. The adjacent linear protruding structures can be arranged with a certain distance D₁ between them. D₁ can range from about 10 nm to about 1000 nm. In some embodiments, D₁ ranges from about 100 nm to about 200 nm. In one embodiment, D₁ is about 140 nm. The linear protruding structures can be arrange along a same direction. A cross-section of each linear protruding structure along the extending direction can be an arc. A height H of the arc can range from about 100 nm to about 500 nm. In some embodiments, H ranges from about 150 nm to about 200 nm. A width D₂ of the arc can range from about 200 nm to about 1000 nm. In some embodiments, D₂ ranges from about 300 nm to about 400 nm. In some embodiments, the cross-section of the linear protruding structure along the extending direction is a semicircle. A diameter of the semicircle can range from about 300 nm to about 400. In one embodiment, the diameter of the semicircle is about 320 nm.

The first electrode 140 can cover the surface of the first semiconductor layer 110 away from the active layer 120. The first electrode 140 can be a single layer structure or a multi-layer structure. A material of the first electrode 140 can be selected from Ti, Ag, Al, Ni, Au, or a combination thereof. The material of the first electrode 140 can also be indium-tin oxide (ITO) or carbon nanotube film. In one embodiment, the first electrode 140 is a two-layer structure comprising a Ti layer with a thickness of about 15 nm and an Au layer with a thickness of about 200 nm.

The second electrode 150 can be electrically connected to the second semiconductor layer 130. In one embodiment, the second electrode 150 is located on the light emitting surface of LED 10. The shape of the second electrode 150 is arbitrary and can be selected according to need. The second electrode 150 can cover a part or the entire surface of the second semiconductor layer 130. A material of the second electrode 150 can be Ti, Ag, Al, Ni, Au, or a combination thereof. The material of the second electrode 150 can also be indium-tin oxide or carbon nanotube film. In one embodiment, the first electrode 140 is a two-layer structure comprising a Ti layer with a thickness of about 15 nm and an Au layer with a thickness of about 100 nm.

Furthermore, a reflector layer (not shown) can be located on a surface of first electrode 140 away from the first semiconductor layer 110. A material of the reflector can be titanium, silver, aluminum, nickel, gold or a combination thereof. The reflector comprises a smooth surface having a high reflectivity. The photons that reach the reflector can be reflected by the reflector. Thus, these photons can be extracted out of the LED 10 to improve the light extraction efficiency of the LED 10.

Referring to FIG. 4, the LED 10 comprises the first three-dimensional nano-structures 134 located on the light emitting surface. Thus, a light having a large incidence angle α (e.g. larger than 23.58°) emitted from the active layer 120, can be transformed into a light having small incidence angle β by the first three-dimensional nano-structures 134. Therefore, the light having small incidence angle β can emit from the LED 10 and the light extraction efficiency of the LED 10 can be improved. Referring to FIG. 5, the light extraction intensity is enhanced by approximately 4.7 times for the LED 10 (curve I) compared with the standard LED (curve II).

Referring to FIGS. 6 and 7, one embodiment of a method for making the LED 10 comprises the following steps:

-   -   (S11), providing a substrate 100 with an epitaxial growth         surface;     -   (S12), forming a first semiconductor layer 110, a active layer         120 and a second semiconductor pre-layer 160 on the epitaxial         growth surface in that order;     -   (S13), applying a patterned mask layer 170 on the second         semiconductor pre-layer 160, forming a number of first         three-dimensional nano-structures 134 on a surface of the second         semiconductor pre-layer 160 away from the active layer 120 and         removing the patterned mask layer 170;     -   (S14), removing the substrate 100 to expose the surface of the         first semiconductor layer 110 away from the active layer 120;     -   (S15), applying a first electrode 140 on the surface of the         first semiconductor layer 110 away from the active layer 120;         and     -   (S16), electrically connecting a second electrode 150 to the         second semiconductor pre-layer 160.

In step (S11), a material of the substrate 100 can be selected according to a material of the first semiconductor layer 110. The substrate 100 can support the first semiconductor layer 110. The substrate 100 can comprise an epitaxial growth surface, which is used to grow the first semiconductor layer 110. The epitaxial growth surface can be a clean and smooth surface. A material of the substrate 100 can be silicon on insulator, LiGaO₂, LiAlO₂, Al₂O₃, Si, GaAs, GaN, GaSb, InN, InP, InAs, InSb, AlP, AlAs, AlSb, AlN, GaP, SiC, SiGe, GaMnAs, GaAlAs, GaInAs, GaAlN, GaInN, AlInN, GaAsP, InGaN, AlGaInN, AlGaInP, GaP:Zn or GaP:N. The first semiconductor layer 110 and the substrate 100 should have a small crystal lattice mismatch and a thermal expansion mismatch. A size, thickness, and shape of the substrate 100 can be selected according to use. In one embodiment, the substrate 100 is a sapphire substrate with a thickness of about 400 μm.

In step (S12), the first semiconductor layer 110 can be grown respectively via a process of molecular beam epitaxy, chemical beam epitaxy, vacuum epitaxy, low temperature epitaxy, selective epitaxial growth, liquid phase deposition epitaxy, metal organic vapor phase epitaxy, ultra-high vacuum chemical vapor deposition, hydride vapor phase epitaxy, or metal organic chemical vapor deposition.

In one embodiment, a material of the first semiconductor layer 110 is Si-doped N-type GaN. The first semiconductor layer 110 is made by a MOCVD method, and a growth of the first semiconductor layer 110 is a heteroepitaxial growth. In the MOCVD method, a nitrogen source gas is high-purity ammonia (NH₃), the carrier gas is hydrogen (H₂), the Ga source gas is trimethyl gallium (TMGa) or triethyl gallium (TEGa), and the Si source gas is silane (SiH₄). The growth of the first semiconductor layer 110 comprises the following steps:

-   -   (a1), placing the substrate 100 into a reaction chamber and         heating the reaction chamber to about 1100° C. to about 1200°         C., introducing the carrier gas, and baking the substrate 100         for about 200 seconds to about 1000 seconds;     -   (a2), growing the low-temperature GaN layer by reducing the         temperature of the reaction chamber to a range from about         500° C. to 650° C. in the carrier gas atmosphere, and         introducing the Ga source gas and the nitrogen source gas at the         same time;     -   (a3), stopping the flow of the Ga source gas in the carrier gas         and nitrogen source gas atmosphere, increasing the temperature         to a range from about 1100° C. to about 1200° C., and         maintaining the temperature for about 30 seconds to about 300         seconds; and     -   (a4), growing the high quality first semiconductor layer 110 by         maintaining the temperature of the reaction chamber in a range         from about 1000° C. to about 1100° C., and reintroducing the Ga         source gas again and the Si source gas.

In step (a2), the low-temperature GaN can be used as a buffer layer (not shown) to grow the first semiconductor layer 110. A thickness of the buffer layer can be less than the thickness of the first semiconductor layer 110. Because the first semiconductor layer 110 and the substrate 100 have different lattice constants, the buffer layer can be used to reduce the lattice mismatch during the growth process, thus the dislocation density of the first semiconductor layer 110 will be decreased.

The growth method of the active layer 120 is similar to the growth method of the first semiconductor layer 110. In one embodiment, the indium source gas is trimethyl indium. The method for growing the active layer 120 comprises the following steps:

-   -   (b1) introducing the hydrogen, nitrogen, and Ga source gas and         maintaining the temperature of the reaction chamber at a         temperature ranged from about 700° C. to about 900° C., and the         pressure of the reaction chamber ranged from about 50 torrs to         about 500 torrs; and     -   (b2) introducing the trimethyl gallium and growing InGaN/GaN         multilayer quantum well film to form the active layer 120.

The second semiconductor pre-layer 160 is grown after the growth of the active layer 120. In one embodiment, the Mg source gas is ferrocene magnesium (Cp₂Mg), and the method comprises the following steps:

-   -   (c1) stopping the flow of the trimethyl gallium and maintaining         the temperature of the reaction chamber in a range from about         1000° C. to about 1100° C., and maintaining the pressure of the         reaction chamber at a pressure ranged from about 76 torrs to         about 200 torrs; and     -   (c2) introducing the ferrocene magnesium and growing P-type         gallium nitride doped with Mg to form the second semiconductor         pre-layer 160.

Referring to FIG. 7, in step (S13), the method for forming the first three-dimensional nano-structures 134 on the surface of the second semiconductor pre-layer 160 away from the active layer 120 can comprise the steps of:

-   -   (S131), forming a mask layer 170 on the surface of the second         semiconductor pre-layer 160 away from the active layer 120;     -   (S132), patterning the mask layer 170 by nanoimprinting method         or etching method;     -   (S133), etching the surface of the second semiconductor         pre-layer 160 to form the first three-dimensional         nano-structures 134; and     -   (S134), removing the mask layer 170.

In step (S131), a material of the mask layer 170 can be ZEP520A, hydrogen silsesquioxane, polymethylmethacrylate, polystyrene, silicon on glass, or other silitriangle oligomers. The mask layer 170 can be used to protect the second semiconductor pre-layer 160, the first electrode 140 and second electrode 150. In one embodiment, the material of the mask layer 170 is ZEP520A.

The mask layer 170 can be formed on the surface of the second semiconductor pre-layer 160, the first electrode 140 and second electrode 150 by spin coating method, slit coating method, slit and spin coating method, or dry film lamination method. In one embodiment, the mask layer 170 is formed by the following steps. First, the surface of the second semiconductor pre-layer 160 is cleaned. Second, a layer of ZEP520A is coated on the second semiconductor pre-layer 160, the first electrode 140 and second electrode 150 by spin coating at a speed of about 500 rounds per minute to about 6000 rounds per minute, for about 0.5 minutes to about 1.5 minutes. Third, the layer of ZEP520A is dried at a temperature of about 140 degrees centigrade to 180 degrees centigrade, for about 3 minutes to about 5 minutes, thereby the mask layer 170 is formed on the second semiconductor pre-layer 160, the first electrode 140, and second electrode 150. A thickness of the mask layer 170 can be in a range of about 100 nm to about 500 nm.

In step (S132), the mask layer 170 can be patterned by electron beam lithography method, photolithography method, or nanoimprint lithography method. In one embodiment, the mask layer 170 is patterned by electron beam lithography. During the patterning process, a number of grooves 172 can be formed in the mask layer 170 to expose the surface of the second semiconductor pre-layer 160. The grooves 172 can be uniformly distributed in the mask layer 170 and spaced from each other. The mask layer 170 between each adjacent two grooves 172 forms a linear wall 174.

A distribution of the linear walls 174 can be the same as a distribution of the first three-dimensional nano-structures 134. The linear walls 174 can be uniformly distributed in the mask layer 170 to form an array. The linear walls 174 in the array can be substantially equidistantly arranged, concentric circularly arranged, or concentric rectangularly arranged. The linear wall 174 can be arranged in a straight line, a curvy line, or a polygonal line. A width of the linear walls 174 can be equal to the width D₂ of the linear protruding structures. The width of the linear walls 174 can range from about 200 nm to about 1000 nm. In some embodiments, the width of the linear walls 174 ranges from about 300 nm to about 400 nm. A distance between adjacent linear walls 174 can be equal to the distance D₁ between adjacent linear protruding structures 114. The distance between adjacent linear walls 174 can range from about 10 nm to about 1000 nm. In some embodiments, the distance between adjacent linear walls 174 ranges from about 100 nm to about 200 nm. In one embodiment, the linear walls 174 are substantially equidistantly arranged and extend along a same direction; the distance between adjacent linear walls 174 is about 140 nm; and the width of the linear walls 174 is about 320 nm.

In step (S133), the process of etching the surface of the second semiconductor pre-layer 160 can be carried out in a microwave plasma system at reaction-ion-etching mode. The microwave plasma system can produce a reactive atmosphere 180. A material of the reactive atmosphere 180 can be chosen according to the material of the second semiconductor pre-layer 160 and the material of the mask layer 170. The reactive atmosphere 180 with lower ions energy, can diffuse to the surface of the second semiconductor pre-layer 160 between adjacent linear walls 174 to etch the surface of the second semiconductor pre-layer 160.

Referring to FIG. 8, for one hand, the reactive atmosphere 180 can etch the second semiconductor pre-layer 160 exposed by the grooves 172 along a first etching direction. The first etching direction is substantially perpendicular to the surface of the second semiconductor pre-layer 160. At the same time, two sidewalls of the second semiconductor pre-layer 160 covered by the linear walls 174 can be formed gradually as the second semiconductor pre-layer 160 is etched along the first etching direction. Thus, the reactive atmosphere 180 can etch the two sidewalls of the second semiconductor pre-layer 160 covered by the linear walls 174 along a second etching direction. The second etching direction can be substantially parallel to the surface of the second semiconductor pre-layer 160. Therefore, the first three-dimensional nano-structures 134 can be formed.

In one embodiment, the reactive atmosphere 180 consists of chlorine gas and argon gas. An input flow rate of the chlorine gas can be lower than an input flow rate of the argon gas. The input flow rate of the chlorine gas can be in a range from about 4 standard-state cubic centimeters per minute to about 20 standard-state cubic centimeters per minute. The input flow rate of the argon gas can be in a range from about 10 standard-state cubic centimeters per minute to about 60 standard-state cubic centimeters per minute. A power of the plasma system can be in a range from about 40 Watts to about 70 Watts. A working pressure of the reactive atmosphere 180 can be a range from about 2 Pa to about 10 Pa. An etching time of the reactive atmosphere 180 can be in a range from about 1 minute to about 2.5 minutes. In one embodiment, the input flow rate of the chlorine gas is about 10 standard-state cubic centimeters per minute; the input flow rate of the argon gas is about 25 standard-state cubic centimeters per minute; the power of the plasma system is about 70 Watts; the working pressure of the reactive atmosphere 180 is about 2 Pa; and the etching time of the reactive atmosphere 180 is about 2 minutes. In step (S134), the first three-dimensional nano-structures 134 can be obtained by dissolving the mask layer 170. The mask layer 170 can be removed by dissolving it in a stripping agent such as tetrahydrofuran, acetone, butanone, cyclohexane, hexane, methanol, or ethanol. In one embodiment, the stripping agent is acetone and the mask layer 170 is dissolved in acetone and separated from the second semiconductor pre-layer 160. The mask layer 170 is removed to form the second semiconductor layer 130.

In step (S14), the substrate 100 can be removed by laser irradiation, etching, or thermal expansion and contraction. The removal method can be selected according to the material of the substrate 100 and the first semiconductor layer 110. In one embodiment, the substrate 100 is removed by laser irradiation. The substrate 100 can be removed from the first semiconductor layer 110 by the following steps:

-   -   S141, polishing and cleaning the surface of the substrate 100         away from the first semiconductor layer 110;     -   S142, placing the substrate 100 on a platform (not shown) and         irradiating the substrate 100 and the first semiconductor layer         110 with a laser; and     -   S143, immersing the substrate 100 into a solvent and removing         the substrate 100.

In step S141, the substrate 100 can be polished by a mechanical polishing method or a chemical polishing method to obtain a smooth surface. Thus the scatting of the laser will decrease. The substrate 100 can be cleaned with hydrochloric acid or sulfuric acid to remove the metallic impurities and oil.

In step S142, the substrate 100 is irradiated by the laser from the polished surface, and the incidence angle of the laser is substantially perpendicular to the surface of the substrate 100. The wavelength of the laser is selected according to the material of the first semiconductor layer 110 and the substrate 100. The energy of the laser is smaller than the bandgap energy of the substrate 100 and larger than the bandgap energy of the first semiconductor layer 110. Thus the laser can pass through the substrate 100 and reach the interface between the substrate 100 and the first semiconductor layer 110. The buffer layer at the interface has a strong absorption of the laser, and the temperature of the buffer layer will be raised rapidly. Thus the buffer layer will be decomposed. In one embodiment, the bandgap energy of the first semiconductor layer 110 is about 3.3 ev, and the bandgap energy of the substrate 100 is about 9.9 ev. The laser is a KrF laser, the wavelength of the laser is about 248 nm, the energy is about 5 ev, the pulse width range is about 20 nanoseconds to about 40 nanoseconds, the energy density ranges from about 400 mJ/cm² to about 600 mJ/cm², and the shape of the laser pattern is square with a size of 0.5 mm×0.5 mm. The laser moves from one edge of the substrate 100 with a speed of 0.5 mm/s. During the irradiating process, the GaN is decomposed to Ga and N₂. The parameters of the laser can be adjusted according to need. The wavelength of the laser can be selected according to the absorption of the buffer layer.

Because the buffer layer has a strong absorption of the laser, the buffer layer can decompose rapidly. However, the first semiconductor layer 110 has weak absorption, so it does not decompose quickly. The irradiating process can be performed in a vacuum or a protective gas environment. The protective gas can be nitrogen, helium, argon or other inert gas.

In step S143, the substrate 100 can be immersed into an acidic solution to remove the Ga decomposed from GaN so that the substrate 100 can be peeled off from the first semiconductor layer 110. The acidic solution can be hydrochloric acid, sulfuric acid, or nitric acid which can dissolve the Ga.

In step S15, the first electrode 140 can be formed via a process of physical vapor deposition, such as electron beam evaporation, vacuum evaporation, ion sputtering, or any physical deposition. Furthermore, the first electrode 140 can also be formed by directly attaching a conductive sheet on the exposed surface of the first semiconductor layer 110 away from the active layer 120. In one embodiment, the first electrode 140 covers the entire surface of the first semiconductor layer 110 away from the active layer 120.

After the first electrode 140 is formed, a step of forming a reflector layer on a surface of first electrode 140 away from the first semiconductor layer 110, can be carried out.

In step (S16), the method for making the second electrode 150 is the same as that of the first electrode 140. The second electrode 150 can be located on the surface of the second semiconductor pre-layer 160 away from the active layer 120. The second electrode 150 can be a continuous layered-structure and cover a part of the surface of the second semiconductor layer 130.

In some embodiments, the step of forming the first three-dimensional nano-structures 134, on the surface of the second semiconductor pre-layer 160, can be carried out after the step of forming the first electrode 140 and second electrode 150.

The method for making the LED 10 has the following advantages. First, by controlling the input flow rates of the chlorine gas and the argon gas, the reactive atmosphere can etch the semiconductor pre-layer along two different etching directions; thus, the first three-dimensional nano-structures can be easily formed on the surface of the semiconductor pre-layer. Second, the method can be carried out at room temperature, thus, the method is simple and low cost.

Referring to FIG. 9, another embodiment of an LED 20 is provided. The LED 20 comprises a first semiconductor layer 210, an active layer 220, a second semiconductor layer 130, a first electrode 140, and a second electrode 150.

The structure of the LED 20 is basically the same as the structure of the LED 10, except that the first semiconductor layer 210 comprises a body 212 and a number of the second three-dimensional structures 214 located on a surface of the body 212 away from the first electrode 140. The second three-dimensional structures 214 can be protruding structures. The protruding structures can protrude out of the surface of the body 212 to form an integrated structure. The second three-dimensional structures 214 can be linear protruding structures, dotted protruding structures, or a combination of linear protruding structures and dotted protruding structures. A cross-section of the linear protruding structure can be triangle, square, rectangular, trapezoidal, arc, semicircle, or other shapes. A shape of the dotted protruding structures can be sphere, ellipsoid, single layer of truncated pyramid, multi-layer of truncated pyramid, single layer of prism, multi-layer of prism, single layer of frustum, multi-layer of frustum or other shapes. In one embodiment, the structures of the second three-dimensional structures 214 are the same as the structure of the first three-dimensional nano-structures 134. That is, a cross-section of each second three-dimensional structure 214 is a semicircle having a diameter of about 320 nm and a distance between adjacent second three-dimensional structures 214 is about 140 nm.

The active layer 220 comprises a number of third three-dimensional structures (not labeled) corresponding to the second three-dimensional structures 214. The third three-dimensional structures can be hollow structures recessed from the surface of the active layer 220 and can correspond to the second three-dimensional structures 214. Thus, the active layer 220 and the first semiconductor layer 210 can be combined without interval. Therefore, a contact surface between the active layer 220 and the first semiconductor layer 210 can be increased, and the electron-hole recombination density can be improved.

The LED 20 can further comprise a number of fourth three-dimensional structures (not labeled) located on the surface of the active layer 220 away from the first semiconductor layer 210. A structure of the fourth three-dimensional structures can be the same as the structure of the second three-dimensional structures 214. Thus, a contact surface between the active layer 220 and the second semiconductor layer 130 can be increased and the electron-hole recombination density can be further improved.

In the LED 20, the surface of the active layer in contact with the first semiconductor layer comprises a number of second three-dimensional nano-structures 214. Thus, the contact area between them can be enlarged. Therefore, the electron-hole recombination density can be further increased and the light extraction efficiency of the LED 20 can be improved.

Referring to FIG. 10, one embodiment of a method for making the LED 20 comprises the following steps:

-   -   (S21), providing a substrate 100 with an epitaxial growth         surface;     -   (S22), forming a first semiconductor pre-layer 260 on the         epitaxial growth surface;     -   (S23), making a number of second three-dimensional         nano-structures 214 on a surface of the first semiconductor         pre-layer 260, away from the substrate 100; thus forming a first         semiconductor layer 210;     -   (S24), applying an active layer 120 and a second semiconductor         pre-layer 160 on the first semiconductor layer 210 in that         order;     -   (S25), applying a patterned mask layer 170 on the second         semiconductor pre-layer 160, forming a number of first         three-dimensional nano-structures 134 on a surface of the second         semiconductor pre-layer 160, away from the active layer 120, and         removing the patterned mask layer 170;     -   (S26), removing the substrate 100 to expose the surface of the         first semiconductor layer 110 away from the active layer 120;     -   (S27), applying a first electrode 140 on the surface of the         first semiconductor layer 110 away from the active layer 120;         and     -   (S28), applying a second electrode 150 electrically connected to         the second semiconductor pre-layer 160.

An embodiment of a method for forming the LED 20 is substantially similar to the method of the LED 10 described above, except that after the first semiconductor pre-layer 260 is formed, a step of forming the second three-dimensional nano-structures 214 on the surface of the first semiconductor pre-layer 260 away from the substrate 100 is further provided. The method for making the second three-dimensional nano-structures 214 can be the same as or different from that of the first three-dimensional nano-structures 134. In one embodiment, the structures of the second three-dimensional nano-structures 214 are the same as the structures of the first three-dimensional nano-structures 134; thus, the method for making the second three-dimensional nano-structures 214 is the same as the method of the first three-dimensional nano-structures 134.

In step S24, the an embodiment of a method for making the active layer 220 is substantially similar to that of the active layer 220 described above, except that the active layer 220 is grown via a horizontal epitaxial growth method. In the embodiment, the substrate 100 with the first semiconductor layer 210 thereon can be located into a horizontal epitaxial growth reactor. A growth direction of the active layer 220 can be controlled by a horizontal growth speed and a vertical growth speed. Thus, the surface of the active layer 220 away from the first semiconductor layer 110 can be planar.

It is to be understood that the above-described embodiment is intended to illustrate rather than limit the disclosure. Variations may be made to the embodiment without departing from the spirit of the disclosure as claimed. The above-described embodiments are intended to illustrate the scope of the disclosure and not restricted to the scope of the disclosure.

It is also to be understood that the above description and the claims drawn to a method may comprise some indication in reference to certain steps. However, the indication used is only to be viewed for identification purposes and not as a suggestion as to an order for the steps. 

What is claimed is:
 1. A method for making a light emitting diode, comprising steps of: providing a substrate having a first surface; forming a first semiconductor layer, an active layer and a second semiconductor pre-layer on the first surface; applying a patterned mask layer on a second semiconductor pre-layer surface, wherein the patterned mask layer comprises a plurality of linear walls aligned side by side, and a groove is defined between each adjacent linear walls to form an exposed portion of the second semiconductor pre-layer surface; etching the exposed portion of the second semiconductor pre-layer surface along a first direction to form two sidewalls in the second semiconductor pre-laver covered by the plurality of linear walls, and etching the two sidewalls along a second direction, and removing the patterned mask layer to form a plurality of three-dimensional nano-structures, wherein the first direction is perpendicular to the second semiconductor pre-layer surface, the second direction is paralleled to the second semiconductor pre-layer surface, and a cross-section of each three-dimensional nano-structure is a convex arc shape; removing the substrate to form an exposed first semiconductor layer surface located away from the active layer; covering the exposed first semiconductor layer surface by a first electrode; and electrically connecting a second electrode with the second semiconductor pre-layer.
 2. The method of claim 1, wherein a width of the groove ranges from about 100 nm to about 200 nm.
 3. The method of claim 1, wherein a distance between adjacent grooves ranges from about 300 nm to about 400 nm.
 4. The method of claim 1, wherein the exposed portion is etched by a reactive atmosphere comprising chlorine gas and argon gas.
 5. The method of claim 3, wherein a chlorine input flow rate is lower than an argon input flow rate.
 6. The method of claim 5, wherein the chlorine input flow rate is in a range from about 4 standard-state cubic centimeters per minute to about 20 standard-state cubic centimeters per minute, and the argon input flow rate is in a range from about 10 standard-state cubic centimeters per minute to about 60 standard-state cubic centimeters per minute.
 7. The method of claim 1, further comprising a step of forming a reflector layer to cover the first electrode.
 8. The method of claim 1, wherein removing the substrate by laser irradiation, etching, or thermal expansion and contraction.
 9. The method of claim 1, wherein the cross-section of each three-dimensional nano-structure is a semicircle having a diameter of about 320 nm.
 10. The method of claim 4, wherein a working pressure of the reactive atmosphere is a range from about 2 Pa to about 10 Pa, and an etching time of the reactive atmosphere is in a range from about 1 minute to about 2.5 minutes. 